Images picked up by image pick-up equipment such as scanners and digital cameras contain noise such as dark current noise, thermal noise, and shot noise due to the characteristics of image pick-up devices and circuits. In order to obtain high-quality images, processing for reducing the noise described above must be performed. If the noise is reduced simply by the use of a low-pass filter, however, significant elements such as an edge with which humans perceive images are lost at the same time, which results in degradation of image quality. Accordingly, technologies for reducing the noise in accordance with the characteristics of the regions of images have been demanded.
As one of such noise reduction technologies, an 6-filter has been proposed (see, for example, Non-Patent Document 1). The ε-filter uses peripheral pixels having a signal difference, less than or equal to a specific threshold, to a notice pixel so as to perform filter processing, which in turn makes it possible to reduce noise while preserving components such as an edge having a large signal difference.
Furthermore, there has been proposed a noise reduction technology using a bilateral filter (see, for example, Non-Patent Document 2). The bilateral filter generates the weighting coefficient of the filter based on a signal difference and a space difference with respect to a notice pixel so as to perform filter processing, which in turn makes it possible to reduce noise while preserving an edge as in the case of the ε-filter. Note that besides the above technologies, there have been proposed ones that reduce noise based on a principle in conformity with the bilateral filter (see, for example, Patent Documents 1 and 2).
However, the ε-filter and the bilateral filter have a problem in that signal components of a background part having a small amplitude are likely to be lost. Therefore, there has been proposed a technology for solving this problem (see, for example, Patent Document 3). Patent Document 3 discloses a technology for performing the weighted addition of an ε-filter output value and a notice pixel value at a ratio corresponding to a space frequency neighboring the notice pixel.
However, even the method described in Patent Document 3 cannot deal with non-additive noise such as shot noise, and it is poor in noise reduction performance with respect to color images. Meanwhile, there has been proposed a technology for reducing non-additive noise in color images (see, for example, Non-Patent Document 3), but this method requires an enormous processing cost for segmenting images and calculating a statistics value.    Patent Document 1: JP-A-2008-205737    Patent Document 2: JP-A-2007-288439    Patent Document 3: JP-A-2002-259965    Non-Patent Document 1: “ε-Separating Non-linear Digital Filter and Its Application,” The transactions A of the Institute of Electronics, Information and Communication Engineers, Vol. J65-A, No. 4, pp. 297-304, 1982 by Hiroshi Harashima, Kaoru Odajima, Yoshiaki Shishikui, and Hiroshi Miyagawa    Non-Patent Document 2: “Bilateral Filtering for Gray and Color Images,” Proc. Sixth Int'l Conf. Computer Vision, pp. 839-846, 1998 by C. Tomasi and R. Manduchi    Non-Patent Document 3: “Automatic Estimation and Removal of Noise from a Single Image,” IEEE Trans. Pattern Analysis and Machine Intelligence, vol. 30, no. 2, pp. 299-314, 2008 by Ce Liu, et. al.